Re: [RFC] KVM: Fix simultaneous NMIs

[Avi Kivity <avi@redhat.com>]

On 09/19/2011 04:54 PM, Marcelo Tosatti wrote:
> >  >>
> >  >>   Yes, due to NMI-blocked-by-STI.  A really touchy area.
> >  >And we don't need the window exit notification then? I don't understand
> >  >what nmi_in_progress is supposed to do here.
> >
> >  We need the window notification in both cases.  If we're recovering
> >  from STI, then we don't need to collapse NMIs.  If we're completing
> >  an NMI handler, then we do need to collapse NMIs (since the queue
> >  length is two, and we just completed one).
>
> I don't understand what is the point with nmi_in_progress, and the above
> hunk, either. Can't inject_nmi do:
>
> if (nmi_injected + atomic_read(nmi_pending)<  2)
>      atomic_inc(nmi_pending)
>
> Instead of collapsing somewhere else?

We could.  It's not atomic though - two threads executing in parallel 
could raise the value to three.  Could do a cmpxchg loop does an 
increment bounded to two.  I guess this is a lot clearer, thanks.

> You'd also have to change
> nmi_injected handling in arch code so its value is not "hidden", in
> complete_interrupts().

Or maybe make raising nmi_injected not decrement nmi_pending.  So:

   nmi_pending: total number of interrupts in queue
   nmi_injected: of these, how many are currently being injected

yes?

-- 
error compiling committee.c: too many arguments to function